The present invention relates to a convergence system for a cathode-ray tube having a plurality of electron guns, particularly a convergence system which is superior in focusing performance and capable of reducing the deterioration of shape of an electron beam spot during convergence correction.
According to a conventional apparatus, as described in Japanese Patent Application No. 50-27966 and the corresponding U.S. Pat. No. 3,430,099, a device for correcting misconvergence of a longitudinal line raster and a device for correcting misconvergence of a lateral line raster are attached to an electron gun of a cathode-ray tube and are separately constructed as magnetic pole pieces and convergence yokes.
In such a prior art construction, the adjustment of convergence between side beams positioned on both sides of a center beam can be made by the aforementioned convergence yokes, but it is impossible to make a convergence adjustment (coma aberration adjustment) between the center beam and the side beams. Particularly, coma aberration of lateral lines is conspicuous on a picture plane, so the correction thereof is absolutely necessary. Generally, in a color cathode-ray tube, the center beam is G (green) beam and the side beams on opposite sides of the center beam are R (red) and B (blue) beams.
In the above prior art, therefore, even in the event of occurrence of coma aberration on a picture plane due to variations in the winding distribution of a deflecting yoke or an error in the mounting position of the electron gun relative to the deflecting yoke, it is impossible to make correction to a satisfactory extent and it has so far been difficult to obtain satisfactory convergence characteristics.